Cable tuners are required to receive a selected channel and convert this to an output intermediate frequency with a minimum signal to noise plus intermodulation level as determined by the type of modulation of the received signal. FIG. 1 of the accompanying drawings illustrates a typical dual conversion cable tuner in which all channel filtering is performed by filters having fixed frequency and bandwidth characteristics. The tuner has an antennae input 1 which receives a broadband radio frequency signal comprising a plurality of channels from a cable distribution network. Little or no filtering is performed on the broadband signal and, in particular, there are no filters with continuously variable bandwidth or centre frequency.
The input 1 is connected to an automatic gain control (AGC) stage 2 whose output is connected to a first frequency changer 3 which performs a block up-conversion of the received channels such that the desired channel is centred on the first high intermediate frequency. The frequency changer 3 comprises a mixer 4 and a local oscillator (LO) 5 controlled by a phase locked loop (PLL) synthesiser 6. The synthesiser 6 is in turn controlled by a controller 7, such as an I2C bus microcontroller which may be implemented as a programmed microcomputer or a state machine. The controller 7 controls the synthesiser 6 so that the local oscillator 5 supplies a signal of the appropriate frequency for converting the desired channel to the first intermediate frequency, which is typically between 1.1 and 1.3 GHz.
The output of the mixer 4 is supplied to a first intermediate frequency filter 8 of bandpass type having a centre frequency at the first intermediate frequency and a defined passband such that the filter 8 passes the desired channel and several adjacent channels to a second frequency changer 9. The second frequency changer 9 comprises a mixer 10 and a local oscillator 11 controlled by a phase locked loop synthesiser 12, which in turn is controlled by the controller 7. The frequency changer 9 performs a down-conversion of the signals received from the filter 8 such that the desired channel is centred on the second intermediate frequency, which is typically between 30 and 50 MHz. The output of the mixer 10 is supplied to a second intermediate frequency filter 13 of bandpass type having a centre frequency at the second intermediate frequency and, typically, a shaped passband characteristic so as to pass the desired channel at the second intermediate frequency and to reject the adjacent channels. The output of the filter 13 is connected to an intermediate frequency amplifier 14 whose output is connected to the intermediate frequency (IF) output 15 of the tuner. The output 15 is connected to an appropriate demodulator (not shown) for recovering the modulation signal. The passband of the filter 13 may be shaped in accordance with the modulation standard of the received signal.
In order to reduce the cost of manufacture, it is desirable to embody as much of the cable turner as possible in an integrated circuit. However, it has not been possible to integrate the filters 8 and 13 into such an integrated circuit because it has not been possible to provide such filters with accurate and repeatable passband and centre frequency characteristics. Also, in the case of tuners embodied as one or more integrated circuits with discrete component filters 8 and 10, the presence of two fixed filters constitutes a substantial portion of the cost of the tuner. Thus, such tuners are too expensive for use in many low cost systems, such as terrestrial analogue receivers and cable modems, which are extremely cost sensitive.
GB 2 335 100 discloses a transceiver including a receiver of the double conversion zero intermediate frequency type. The receiver has a first intermediate frequency filter in the form of a voltage controlled bandpass filter because of the zero intermediate frequency mode of operation, the local oscillator of the second frequency changer is at the middle of the channel selected for reception following conversion in the first frequency changer. The centre frequency of the first intermediate frequency filter is controlled by a phase locked loop having a phase sensitive detector which compares the output of the second local oscillator with a second local oscillator signal leaking back through the first intermediate frequency filter. The filter is controlled so as to remove any phase difference so that the passband is then centred on the frequency of the second local oscillator and hence at the centre of the channel selected for reception.
GB 2 294 602 discloses a cellular telephone of double conversion type in which a second local oscillator is phase-locked to the received signal during the normal reception mode. The telephone has second intermediate frequency filters in the form of low pass filters with a cut-off frequency which is switched to give a narrower passband when the telephone is in a scanning mode.
GB 2 270 223 discloses a receiver which provides periodic calibration of a voltage controlled intermediate frequency filter. An externally generated reference signal is modulated by frequency-modulating the local oscillator signal to the mixer. Signal strength measurements are used as the basis for adjusting the intermediate frequency filter to provide maximum sensitivity while rejecting adjacent channels as much as possible.
GB 2 101 432 discloses an FM (frequency modulation) receiver having a phase locked loop demodulator. The phase locked loop has a loop filter whose bandwidth is switched according to the received signal strength.
EP 0 863 623 discloses the use of an intermediate frequency filter in the form of a gyrator. The output of the filter is supplied to a frequency discriminator, which controls the filter so that its centre frequency tracks changes in the centre frequency of the signal supplied to the filter so as to compensate for frequency drifting resulting from changes in temperature in respect of the signal supplied to the filter.
WO 99/59256 discloses an AM (amplitude modulation) receiver formed as a single integrated circuit. The receiver has intermediate frequency filtering characteristics which are adjustable and are determined by a phase locked loop arrangement.